1. Field of the Invention
The invention relates to an alloy that comprises lead, tin, silver, and bismuth for use in the positive grids of lead-acid batteries. In particular, the alloy of the present invention may be used to form thin grids by any method, including expanded metal processing and book mold casting. The alloys of the present invention may provide one or more of the following properties: grids may be formed without resort to extraordinary measures, harden relatively rapidly, relatively stable (i.e., enable a battery containing them to provide relatively long term service), and relatively easy recycleability.
2. Description of Related Technology
Modern storage batteries require a relatively large numbers of grids, which requires that the grids be particularly thin. These high performance batteries allow for relatively high voltages, amperages, rapid recharge, or a combination thereof, which makes them particularly useful for automobile starting batteries, full electric and hybrid electric vehicles, and stationary batteries for uninterruptible power service or telecommunications service.
Production of thin grids whether conventional book mold cast, continuously cast, continuously cast strip followed by expansion or direct continuous cast followed by rolling, typically entails handling the grid or the strip at relatively high temperatures. The thinner the grid or strip, the more difficult it is to handle the grid or strip at such temperatures. Typical production processes rapidly decrease the temperature of the grid or strip with air cooling, water cooling, or water-cooled trim dies and platens depending on the process. The enhanced reduction in temperature has been used for lead-calcium alloy grids because they tend to be weak at elevated temperatures and a rapid reduction in temperature tends to counter deformations or thickness changes due to inadequate hardness. Despite rapid cooling to room temperature, many grid materials produced from low calcium, lead-based alloys tend to be difficult to handle due to inadequate hardness even at room temperature.
In addition to hardness, the physical dimensions of grids/strips also affect the amount of handling/processing a grid/strip is able to acceptably withstand. In general, grids having a thickness of at least 0.060 inches (1.524 mm) typically have enough mass so that they are better able to withstand handling/processing despite having low mechanical properties. Thus, such “thick” grids typically may be cooled to room temperature more slowly than grids having a thickness that is less than 0.060 inches (1.524 mm) (i.e., “thin” grids). Also, thick grids typically withstand the handling associated with pasting more readily than thin grids.
Certain mechanical properties of lead-calcium grid alloys depend, not only on temperature, but also on aging. Specifically, after being reduced to room temperature, the hardness of such alloys tends to be greater after a period of time has lapsed than when it initially reached room temperature.
Lead-calcium-based alloys largely replaced lead-antimony-based alloys as the materials of choice for positive grids of both automobile and stationary lead-acid batteries for a variety of reasons. Lead-antimony alloys were replaced primarily because they tend to corrode more rapidly than lead-calcium alloys. This corrosion is detrimental because it tends to result in the release of antimony, which during a recharge process, tends to migrate to the negative plate where it causes a loss of water from the electrolyte, particularly when exposed to relatively hot environments. In contrast, lead-calcium alloys tend to be significantly resistant to water loss during service and, as a result, they are widely used to make grids for “maintenance-free” or sealed lead-acid (SLA) batteries.
Lead-calcium alloys have also be widely utilized because they typically have a very low freezing range and are capable of being processed into positive and negative grids by a variety of grid manufacturing processes, such as conventional book mold casting, rolling and expanding, continuously casting followed by expansion or punching, continuous grid casting, and continuous grid casting followed by rolling. Continuous grid manufacturing processes are particularly desirable because they typically decrease production costs associated with battery grid and plate production.
The early lead-calcium alloys typically contained a relatively high calcium content (e.g., 0.08% or higher) and relatively low tin content (e.g., 0.35-0.5%). Advantageously, positive grids produced from these alloys hardened rapidly and could be handled and pasted into plates easily. Specifically, these alloys, because of the high calcium content, tend to form Pb3Ca precipitates over Sn3Ca precipitates and, although the Pb3Ca precipitates tends to harden the alloy, they tend to result in increased corrosion and growth of positive grids in high temperature applications (e.g., newer, more aerodynamic automobiles with less cooling of the battery by flowing air). To address this problem, lead-calcium alloys were developed that contain lower calcium concentrations and other metals added to the alloy (e.g., U.S. Pat. Nos. 5,298,350; 5,434,025; 5,691,087; 5,834,141; 5,874,186; as well as DE 2,758,940). The grids produced from these alloys, however, are not without problems. The very low calcium contents (0.02-0.05%) generally utilized in the grid alloys produce grids which are very soft, difficult to handle, and harden very slowly. To utilize grids produced from these alloys, the cast material is usually stored at room temperature for long periods of time or artificially aged at elevated temperatures to bring the material to sufficiently high mechanical properties to be handled in a pasting or expander/paster machine.
Low-calcium alloys typically also contain tin at a relatively low amount and silver at a relatively high amount and these alloys tend to be very corrosion-resistant. Nevertheless, in addition to the above-described handling issue, these alloys also usually require special procedures in order to be made into a battery plate. Specifically, a grid is typically pasted with a mixture of leady lead oxide, sulfuric acid, water and some additives. After pasting, the plates are cured to permit the paste (active material of the battery) to firmly adhere to the battery grid so that there is sufficient electrical contact between the grid and the active material. Unfortunately, to cure the plates, the grids must be corroded so that the paste adheres to the grid, which requires manufacturers to resort to significant effort and cost to corrode the corrosion-resistant grids. Examples of such efforts include treating the grids for long periods of time in hot steam environments to produce a corrosion film on the grid surface; treating the surface of the grids with alkaline reagents, peroxides, or persulfates; and long curing times at high temperature and humidity for as long as five days. Despite these efforts, the most common failure mechanism of batteries using such alloys is the disengagement of active material disengagement from the positive grid, not positive grid corrosion.
Such low Ca-low Sn-high Ag lead-based alloys have yet another problem that is due principally to the relatively low tin content (e.g., 0.3-0.6%). Specifically, the low tin contents permit the formation of non-conductive oxide layers between the grid and active material when the battery becomes discharged. The electrical resistance of these oxide products may prevent adequate charge acceptance during recharge of the battery if it becomes discharged, thus resulting in premature failure.
In view of the foregoing, a need exists for lead-based alloys for use in the production of grids for lead-acid batteries, in general, and positive grids, in particular, and having one or more of the following characteristics, abilities, and/or uses: resistance to corrosion at relatively high temperatures such as those found in automobile engine compartments; capable of being used to produce thin grids by any method desired (e.g., continuously cast-expansion or punched, roll-expansion, continuously cast, continuously cast-rolled, or conventional book mold casting); hardens relatively rapidly so that the grid may be utilized in the production of battery plates within a relatively short period of time after production; that may be used without excessively long aging periods or without resorting to artificial aging; certain pastes adhere to the grid surface without curing; resistance to formation of non-conductive oxide layers between the grid and active material when a battery containing the grid is discharged; a degree of creep resistance and mechanical properties that allow the battery grid to resist the effects of elevated temperatures; and a grain structure stability resulting in reduced corrosion and the improved retention of the mechanical properties and active material at elevated temperatures.